From The Collaborative International Dictionary of English v.0.48 :
Gunwale \Gun"wale\, n. [Gun + wale. So named because the upper
guns were pointed from it.] (Naut.)
The upper edge of a vessel's or boat's side; the uppermost
wale of a ship (not including the bulwarks); or that piece of
timber which reaches on either side from the quarter-deck to
the forecastle, being the uppermost bend, which finishes the
upper works of the hull. [Written also gunnel.]
[1913 Webster]
From WordNet (r) 3.0 (2006) :
gunwale
n 1: wale at the top of the side of boat; topmost planking of a
wooden vessel [syn: gunwale, gunnel, gun rest]